Method and device for crucible-free floating-zone melting using a variable-diameter heating coil

ABSTRACT

Method of floating-zone melting includes surrounding a crystalline rod with an annular induction coil of given diameter at an end of the rod having a transition portion tapering outwardly from a seed crystal of considerably smaller thickness than that of the rod, energizing the coil to inductively heat the rod end to molten temperature, and relatively displacing rod and coil along the transition portion and simultaneously gradually increasing the coil diameter. Device for carrying out the foregoing method includes an induction coil of flexible material and a mechanical adjusting device actuable for varying the diameter of the coil.

I United States Patent I 3,607,138

[72] Inventor Wolfgang Keller [56] References Cited t d. Germany UNITED STATES PATENTS Q12;- 31%? 1968 2,905,798 9/1959 Freutel 23/273 1 3,101,165 8/1963 Barkley et a1... 219/l0.79 [451 Meme! 3 331 93s 7/1967 McDanels 219/10 79 [73] Assignee Siemens Aktiengesellschaft Bern" -am] Munich Germany 1 FOREIGN PATENTS [32] Priority Apr. 21, 1967 650,184 2/1951 Great Britain 2l9/10.75 [33] Germany Prima Examiner-Norman Yudkofi [3 l 1 s 109450 Assista it Examiner-R. T. Foster Att0rney.rCurt M. Avery, Arthur E. Wilfond, Herbert L.

Lerner and Daniel J. Tick [54] METHODAND DEVICE FOR CRUCmLE-FREE ABS'glIACT: Metlllrod ofdfloatLng-zone nlielt ncgi includes slur;

FLOATINGJONE MELTING USING A VARIABLE roun ing a crysta me ro wit a: annu ar 1n uction co1 o DIAMETER HEATING COIL given diameter at an end of the re having a Il'Zl'lSltlOn portion 6 Claims, 2 Drawing Fissta ermg outwardly from a seed crystal of COl'lSl erably smaller thickness than that of the rod, energizing the coil to inductive- [52] U.S. Cl 23/301 SP, ly heat the rod end to molten temperature, and relatively dis- 23/273 SP,219/ l0.75,219/l0.43, 219/ 10.79 placing rod and coil along the transition portion and simul- [51] lnt.Cl. HOSb 9/02, taneously gradually increasing the coil diameter. Device for B01 j 17/18 carrying out the foregoing method includes an induction coil [50] Field 01 Search 23/273; of flexible material and a mechanical adjusting device actuable for varying the diameter of the coil.

121 3 54 ,F- f; 1 11. LnJ LnJ PATENTEU SEP21 Ian Fig. .1

METHOD AND DEVICE FOR CRUCIBLE-FREE FLOATING-ZONE MELTING USING A VARIABLE- DIAMETER HEATING COIL My invention relates to method and device for the cruciblefree floating-zone melting of a crystalline rod, particularly a semiconductor rod, to an end of which a seed crystal, having a considerably smaller thickness than that of the rod, is fused by means of an annular induction heating coil surrounding the rod.

What is meant and to be understood herein by a seed crystal having a considerably smaller thickness than that of the rod is that the seed crystal has a cross section that is onetenth or less of the cross section of the crystalline rod. The production of crystalline rods, for example silicon rods, with very great thicknesses of 30 to 50 mm. or more requires a seed crystal having the foregoing relative dimensions. When the monocrystalline seed crystal is accordingly fused to the crystalline rod of relatively much greater cross section, various difficulties occur, because the induction heating coil, whose inner diameter is accommodated to the cylindrical part of the crystalline rod, is then very poorly coupled with the considerably thinner seed crystal. The poor coupling causes an increased scattering of the magnetic field. Consequently, a relatively long and accordingly unstable molten zone is formed when melting the seed crystal. These difficulties can be avoided in part by increasing the current energizing the induction heating coil when fusing the seed crystal to the crystalline rod and also in the case where a conical or tapering transition portion is formed between the seed crystal and the main cylindrical portion of the crystalline rod. The unstable melting zone must then in addition be frequently supported or levitated from below, for example by means of a middlefrequency field. For this purpose, besides an additional levitating coil, a high frequency electrical energy source is required that provides an efficiency that is considerably higher than that afforded for the zone-melting of the cylindrical portion of the crystalline rod.

It is accordingly an object of my invention to provide method and device for crucible-free floating-zone melting a crystalline rod which avoids the foregoing disadvantage of the heretofore known methods and devices of this general type. More particularly, it is an object of my invention to provide such method and device which will afford good coupling between the heating coil, on the one hand, and the seed crystal and crystalline rod, respectively, on the other hand.

With the foregoing and other objects in view, I accordingly provide method of the foregoing type which comprises surrounding the end of the crystallizing rod with an induction heating coil of given diameter, electrically energizing the coil at the end thereof fused or being fused to the seed crystal, and relatively displacing the rod and the coil in the direction of the rod axis along a substantially conical transition portion of the rod tapering outwardly from the seed crystal and simultaneously gradually increasing the diameter of the coil in accordance with the increasing diameter of the tapering transition portion.

Consequently, an optimum use of the high frequency electrical energy that is available is assured in all phases of the zone melting process, and the molten zone being passed through the crystalline rod is always kept short in the axial direction of the rod and accordingly stable.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as method and device for crucible-free floating-zone melting a crystalline rod, particularly a semiconductor rod, it is nevertheless not intended to be limited to the details shown, since various modifications may be made in the method and structural changes in the device without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction of the device of the invention and method of its operation, however, together with additional objects and advantages thereof will be best understood from the following description when read in connection with the accompanying drawing, wherein:

FIG. 1 is a top plan view of the device of my invention showing an induction heating coil whose ends are secured to a suitable device for adjusting the diameter of the coil to the cross section of a crystalline rod; and

FIG. 2 is a front elevational view, reduced in size, of the device of FIG. 1 showing therein a fused crystalline rod and seed crystal supported by end holders.

Referring now to the drawing, there is shown an annular induction heating coil 1 surrounding a crystalline rod R of semiconductor material, such as silicon, for example, fused to a monocrystalline seed crystal, respectively supported vertically in end holders RH and SH. The coil 1 consists of a single winding whose ends 2 and 3 cross one another and are secured to a tentering frame 4. The coil 1 consists of a flexible tubular material, for example an alloy of beryllium and copper, and is cooled by a fluid such as water, for example, which is passed through the hollow interior thereof. Thus, the ends 2 and 3 of the coil I serve both for respectively supplying the coolant fluid to and discharging it from the winding of the coil as well as to conduct a high frequency current therethrough from a nonillustrated electrical energy source. The tentering frame 4 is formed of material that is not ferromagnetic, such as brass for example, or of a nonmagnetic stainless steel. The tentering frame 4 can be fluid-cooled in a manner similar to the induction heating coil 1 by providing suitable passages therein transversible by a fluid coolant such as water for example. The tentering frame 4 is formed of a pair of balanced bow members 6a and 6b pivotable about a common pivot axis 5. The ends 2 and 3 of the induction heating coil 1 are fastened by straps 12a and 12b to insulating members 7a and 7b of ceramic material for example, such as aluminum oxide, which are mounted for example by screws, as illustrated, on each of the bow members 6a and 6b of the tentering frame 4. A pair of spreading jaws 8a and 8b are also turnably mounted on the pivot axis 5 and are respectively connected to the bow members 6a and 6b. The spreading jaws 8a and 8b can be more or less sharply spread apart and turned about the pivot axis 5 by means of a conical adjusting pin 9 insertable therebetween. The adjusting pin 9 can, as illustrated in FIG. 1, be constructed as a shaft passing through a shaft bearing 10. Depending upon the apex angle and length of the conical portion 1 1 of the adjusting pin or shaft 9, the tentering frame 4 is spread more or less, thereby applying tension to the ends 2 and 3 of the coil 1 so as to vary thereby correspondingly the inner diameter of the coil winding.

In an example, the composition of the aforementioned beryllium-copper alloy was 2 percent to 3 percent Be and the remainder Cu. The diameter of the crystalline rod and seed crystal were respectively 35 mm. and 5 mm. The largest inner diameter of the induction heating coil was 38 mm. and the smallest was 25 mm.

It is of course quite apparent that details of the herein described and illustrated embodiment of the device of my invention can be varied within the scope of the invention. Thus the diameter of the induction heating coil 1 can be varied by means of any suitable mechanical adjusting device other than the device actually illustrated and described. It is of importance only that the induction heating coil 1 be made of a material that has the required flexibility even in the stressed condition thereof or while subjected to current flow therethrough. This requirement is satisfied to a great extent by the aforementioned beryllium-copper alloy. In order to increase the restoring force of the coil 1 when the conical portion 11 of the adjusting pin 9 is withdrawn from between the jaws 8a and 8b a tension spring 13. is connected between the jaws 8a and 8b.

1 claim:

1. Method of floating-zone melting a crystalline rod which comprises surrounding the crystalline rod with a winding of an annular induction heating coil of given reduced diameter at an end of the rod having a transition portion tapering outwardly from a seed crystal of considerably smaller thickness than that of the rod, electrically energizing the coil so as to inductively heat the end of the rod to molten temperature, and relatively displacing the rod and the coil in the direction of the rod axis along the tapering transition portion and simultaneously gradually increasing the diameter and circumferential length of the coil winding in accordance with the increasing diameter of the transition portion.

2. Method according to claim 1 wherein the seed crystal is aligned with the rod adjacent the end thereof, and which includes melting the end of the rod so as to fuse the rod to the seed crystal and simultaneously form the tapering transition in the rod by energizing the heating coil winding surrounding the end of the rod.

3. Method according to claim 1, wherein the rod has a substantially rounded portion at the end thereof and which includes aligning the seed crystal with the rod adjacent the substantially rounded end thereof, melting the end of the rod so as to fuse the pod to the seed crystal and simultaneously form the i tapering transition in the rod by energizing the heating coil winding surrounding the end of the rod.

4. Device for carrying out a method of floating-zone melting a crystalline rod comprising an induction heating coil-consisting of flexible material, and a mechanical-adjusting device connected to said coil and actuable for applying varying stress to a winding of said coil so as to vary the diameter and circumferential length thereof.

5. Device according to claim 4 wherein said mechanical adjusting device is a tentering frame, and the ends of said coil winding extend across one another and are secured to said tentering frame.

6. Device according to claim 5 wherein said tentering frame comprises a pair of bow members pivotable about a common axis, said ends of said coil winding being respectively c0nnected to said bow members. 

2. Method according to claim 1 wherein the seed crystal is aligned with the rod adjacent the end thereof, and which includes melting the end of the rod so as to fuse the rod to the seed crystal and simultaneously form the tapering transition in the rod by energizing the heating coil winding surrounding the end of the rod.
 3. Method according to claim 1, wherein the rod has a substantially rounded portion at the end thereof and which includes aligning the seed crystal with the rod adjacent the substantially rounded end thereof, melting the end of the rod so as to fuse the pod to the seed crystal and simultaneously form the tapering transition in the rod by energizing the heating coil winding surrounding the end of the rod.
 4. Device for carrying out a method of floating-zone melting a crystalline rod comprising an induction heating coil consisting of flexible material, and a mechanical-adjusting device connected to said coil and actuable for applying varying stress to a winding of said coil so as to vary the diameter and circumferential length thereof.
 5. Device according to claim 4 wherein said mechanical adjusting device is a tentering frame, and the ends of said coil winding extend across one another and are secured to said tentering frame.
 6. Device according to claim 5 wherein said tentering frame comprises a pair of bow members pivotable about a common axis, said ends of said coil winding being respectively connected to said bow members. 